1. Field of the Invention
The present invention relates generally to a wireless charging network, and more particularly, to a method for generating a load variation used for detecting a wireless power receiving unit in wireless charging, allowing a wireless power transmitting unit to detect a wireless power receiving unit that has entered a wireless charging network.
2. Description of the Related Art
Mobile terminals such as a mobile phone, a Personal Digital Assistant (PDA), etc., are driven with rechargeable batteries, and the battery of the mobile terminal is charged using a separate charging apparatus. In general, the charging apparatus and the battery each have external contact terminals, and the charging apparatus and the battery are electrically connected to each other using the contact terminals.
However, since the contact terminal protrudes outward in such a contact type charging scheme, the contact terminal is easily contaminated by a rogue object and thus battery charging might not be performed correctly. Further, battery charging might also not be performed correctly when the contact terminal is exposed to moisture.
Recently, a wireless charging or a non-contact charging technology was developed and used for electronic devices to solve the above-mentioned problems.
Such a wireless charging technology employs wireless power transmission/reception, and corresponds to, for example, a system in which a battery can be automatically charged if the battery is laid on a charging pad without the need of a wired connection between the mobile phone and a separate charging connector. Examples of wireless charging technology include the wireless electrical toothbrush and the wireless electric shaver. Accordingly, electronic products are charged in a waterproof manner through wireless charging, and the portability of electronic products is increased since there is no need for a wired charging apparatus. Therefore, the number of technologies using wireless charging technology is expected to increase significantly, especially in the coming age of electric cars.
Wireless charging technology largely includes an electromagnetic induction scheme using a coil, a resonance scheme, and an RF/microwave radiation scheme for converting electrical energy to either an RF or microwave signal and transmitting the RF or microwave signal.
Presently, electromagnetic induction schemes are not mainstream, but it is expected that the day will come when all electronic products will be wirelessly charged, anytime and anywhere. Based on recent successful experiments on transmitting power wirelessly dozens of meters using microwaves, wired charging technology is expected to disappear in the near future for use at home and abroad.
A power transmission method using electromagnetic induction transmits electrical power between a primary coil and a secondary coil. When a magnet is moved in a coil, a current is induced in the coil according to the rate of change of the magnetic field. The induction current then generates a magnetic field at a transferring end to generate energy at a reception end. The phenomenon is referred to as magnetic induction, and the electric power transmission method using magnetic induction has a high energy transmission efficiency.
With respect to the resonance scheme, Prof. Soljacic of the Massachusetts Institute of Technology (MIT) announced a system in which electricity is wirelessly transferred using an electric power transmission principle of the resonance scheme based on a coupled mode theory even if a device to be charged is separated from a charging device by several meters (m). A wireless charging system of an MIT research team employed the concept of resonance (e.g., the tendency of a tuning fork oscillating at a particular frequency to cause a wine glass next to the tuning fork to oscillates at the same frequency to make an electromagnetic wave containing electrical energy resonate instead of making sounds resonate. The resonated electrical energy is transferred only when there is a device having a resonance frequency, and only the portion of the resonating electrical energy that is are being used is reabsorbed into an electromagnetic field instead of beingbroadcast. Therefore, the resonating electrical energy would not affect surrounding devices or people, unlike other electromagnetic waves.
There is a method for detecting a change in impedance in which a wireless power transmitting unit (PTU) determines that a wireless power receiving unit (PRU) is put thereon.
However, when detecting a load (i.e., an impedance), the PTU is very likely to falsely detect a change in impedance when a threshold for detecting a change in impedance is set too low. On the other hand, if the threshold for detecting a change in impedance is set too high, the PTU is very likely to fail to detect a change in impedance when an object's change in impedance is insignificant.
In addition, the PTU may not accurately detect a variation in load, if there is only a small change in impedance between a case where no PRU is put on the PTU and another case where a PRU is put on the PTU. In other words, the difference in impedance between the two cases should be sufficiently large in order for the PTU to accurately detect a variation in load.
In the conventional impedance detection method, a PTU may hardly detect a variation in load, because a change in power due to the variation in load is insignificant, even though a resistance varies when a PRU is put on the PTU. In addition, a point at which there is no change in reactance may exist on the PTU.
Therefore, there is a need for a method capable of accurately detecting a PRU when the PRU is placed on a PTU.